Small-size high-speed transmission system for use in microturbine-powered aircraft

ABSTRACT

A transmission system that is used in conjunction with a microturbine engine for propelling an aircraft body, such as a propeller-based fixed-wing aircraft or a rotor-based vertical lift aircraft, or for a wide variety of other applications. The output shaft of the microturbine engine preferably operates at a rotational speed in a range between 72,000 RPM and 150,000 RPM with an output power between 150 HP and 5 HP (and most preferably operates in an extended range between 50,000 RPM and 200,000 RPM with an output power between 200 HP and 5 HP). The two reduction stages provide a reduction ratio preferably having a value of at least 19, and most preferably greater than 24. The transmission system is of small-size preferably having a maximum diameter less than twelve inches. The two stages of the transmission system may comprise any one (or parts of) of a number of configurations, including an in-line lay shaft configuration, an in-line star-star configuration, an offset star-spur configuration, an offset compound idler configuration, an inline traction-internal gear configuration, and an inline traction-planetary gear configuration. Preferably, the input stage of the transmission system is self-equilibrating such that first shaft can be supported without bearings and is operably coupled to the output shaft of the microturbine engine by an outside diameter piloted spline coupling mechanism. For vertical lift applications, a single traction stage along with a bevel gear assembly or other shaft transmission mechanism can be used to provide the necessary RPM reduction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to the speed reduction systems fortransmission of power from a gas turbine engine to a rotating driveelement of an aircraft. More particularly, this invention relates tospeed reduction systems for transmission of power from a small-sizehigh-speed gas turbine engine to a slower-speed rotating drive elementof an aircraft.

2. State of the Art

Small low-cost unmanned air vehicles (UAV's) have been developed anddeployed to carry out a variety of military roles such as reconnaissanceand attack missions. Currently, intermittent combustion piston enginesof 100 HP (or less) power all of the low speed UAV aircraft. Most ofthese engines drive propellers without the need for a gearbox. However,these engines burn gasoline, which is highly flammable and thusundesirable for field service operations. Piston engines also haveundesirable vibration characteristics and are difficult to start in coldweather operations.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an improvedlightweight and small-sized propulsion system suitable for use in a UAV.

It is another object of the invention to provide an improved lightweightand small-sized propulsion system which consumes fuel of lowflammability, such as JP-8 fuel.

It is yet another object of the invention to provide a lightweight andsmall-sized propulsion system with improved vibration characteristics.

It is still another object of the invention to provide a lightweight andsmall-sized propulsion system with improved starting capabilities incold weather operations.

It is another object of the invention to provide a small and lightweighttransmission system that is suitable for use with a high speedmicroturbine to provide the necessary RPM reduction for aircraftpropulsion applications.

It is another object of the invention to provide such a transmissionsystem that is suitable for use with a microturbine whose output shaftis operating in a range between 72,000 RPM and 150,000 RPM with anoutput power between 150 HP and 5 HP, and preferably operating in anextended range between 50,000 RPM and 200,000 RPM with an output powerbetween 200 HP and 5 HP.

It is a further object of the invention to provide such a transmissionsystem that provides a reduction ratio of at least 19 and preferablygreater than 24, which is suitable for UAV aircraft applications.

It is a further object of the invention to provide such a transmissionsystem whose maximum diameter is less than 12 inches.

It is still another object of the invention to provide such atransmission system that avoids bearings for supporting the input shaftof the transmission system.

In accord with these objects, which will be discussed in detail below,an unmanned air vehicle (UAV) is provided which uses a microturbineengine for propelling an aircraft via a transmission system (or for awide variety of other applications). The transmission system has a firstshaft operably coupled to an output shaft of the microturbine engine,which may operate at a rotational speed in a range of between 72,000 RPMand 150,000 RPM with an output power between 150 HP and 5 HP (andpreferably operates in an extended range between 50,000 RPM and 200,000RPM with an output power between 200 HP and 5 HP). Two reduction stagesdrive a second shaft at a reduced rotation speed with respect to thefirst shaft. The two reduction stages provide a reduction ratiopreferably having a value of at least 19, and most preferably greaterthan 24. The transmission system is of small-size preferably having amaximum diameter less than twelve inches. The second shaft of thetransmission system is operably coupled to a propeller for propelling anaircraft body, such as a fixed-wing aircraft body. The two stages of thetransmission system may comprise any one (or parts of) of a number ofconfigurations, including an in-line lay shaft configuration, an in-linestar-star configuration an offset star-spur configuration, an offsetcompound idler configuration an inline traction-internal gearconfiguration and an inline traction-planetary gear configuration.

According to one embodiment of the invention, the input stage of thetransmission system (and propulsion systems based thereon) isself-equilibrating such that first shaft can be supported withoutbearings.

According to another embodiment of the invention, an outside diameterpiloted spline coupling mechanism couples the output shaft of themicroturbine engine to the first shaft of the transmission system.

According to another embodiment of the invention, a single tractionstage along with a bevel gear assembly or other shaft transmission meanscan be used to provide the necessary RPM reduction between the output ofthe microturbine engine and the rotor of a vertical lift aircraft.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a propeller-based propulsion system,including a microturbine engine and transmission system in accordancewith the present invention

FIG. 1B is a pictorial illustration of an exemplary fixed-wing UAV thatemploys the propeller-based propulsion system of FIG. 1A in accordancewith the present invention.

FIG. 2 is a cross-section depicting a first illustrative embodiment ofthe transmission system of FIG. 1A, which is realized by a two stagein-line lay shaft configuration

FIG. 3 is a cross-section depicting a second illustrative embodiment ofthe transmission system of FIG. 1A, which is realized by a two stagein-line star-star configuration.

FIG. 4 is a cross-section depicting a third illustrative embodiment ofthe transmission system of FIG. 1A, which is realized by a two stageoffset star-spur configuration.

FIG. 5A is a schematic diagram depicting a fourth illustrativeembodiment of the transmission system of FIG. 1A, which is realized by atwo stage offset compound-idler configuration.

FIG. 5B is a cross-section depicting the compound-idler configuration ofFIG. 5A.

FIG. 6A is a schematic diagram depicting a fifth illustrative embodimentof the transmission system of FIG. 1A, which is realized by a two stagetraction-internal gear configuration.

FIG. 6B is a cross-section depicting the traction-internal gearconfiguration of FIG. 6A.

FIG. 7 is a cross-section depicting a sixth illustrative embodiment ofthe transmission system of FIG. 1A, which is realized by a two stagetraction-planetary gear configuration.

FIGS. 8A and 8B are schematic diagrams illustrating the principles of aself-equilibrating reduction stage.

FIG. 9 is a cross-section illustrating an outside diameter pilotedspline coupling mechanism that couples the output shaft of themicroturbine engine to the input shaft of the transmission system inaccordance with the present invention.

FIG. 10 is a cross-section illustrating a coupling mechanism thatcouples the output shaft of the transmission system to a propeller for afixed-wing propeller-driven UAV.

FIG. 11A is a schematic diagram of a rotor-based propulsion system,including a microturbine engine and transmission system in accordancewith the present invention.

FIG. 11B is a pictorial illustration of an exemplary vertical lift UAVthat employs the rotor-based propulsion system of FIG. 11A in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Recently, small-size turbine engines (referred to herein as“microturbines”) have been developed primarily for the radio controlmodel airplane market. An example of such a microturbine engine isdisclosed in detail in U.S. Pat. No. 5,727,378 to Seymour. Suchmicroturbines, when used in conjunction with heavy jet fuel (such asJP-8 fuel) provide a highly advantageous propulsion system for small,low-cost UAVs. The advantages afforded by such microturbines includelighter weight, use of less flammable fuels, higher reliability andreduced vibrations.

However, microturbines operate at very high rotational speeds, typicallyin the range between 72,000 RPM and 150,000 RPM with an output powerbetween 150 HP and 5 HP. Such rotational speeds and output power mayextend to a range between 50,000 RPM and 200,000 RPM with an outputpower between 200 HP and 5 HP. UAV aircraft operate at much slowerpropeller rotational speeds, typically on the order of 3700 RPM to 4500RPM. These constraints result in a required reduction ratio from thetypical microturbine engine RPM to the propeller RPM on order of 28:1 to24:1.

Because of the high output speeds of the microturbine (which is 2.5 to10 times higher that current state of the art production turbineengines), current transmission designs for turbine engines do notprovide the necessary RPM reduction, nor do such designs integrate thenecessary RPM reduction functionality into a low-cost, small andlightweight design that is suitable for use in UAVs.

Turning now to FIG. 1A, there is shown a power plant 10 suitable for usein propelling a fixed-wing aircraft, such as an unmanned fixed-wingaircraft 30 shown in FIG. 1B. The power plant 10 includes a microturbineengine 12 with an output shaft 14. A coupling mechanism 16 couples theoutput shaft 14 to the input shaft 18 of a transmission system 20. Thetransmission system 20 operates to reduce the speed of the output shaft14 of the microturbine engine 12 at its own output shaft 22. The outputshaft 22 of the transmission system is coupled to a propeller 24 by acoupling mechanism 26. The propeller 24, when driven by the microturbineengine 12 and transmission system 20, provides thrust that propels anaircraft body, such as the body of the fixed-wing aircraft 30 of FIG.1B. Note that in the configuration shown, the transmission system 20 andpropeller 24 are disposed on the intake side of the microturbine engine12. This configuration allows the transmission system to be cooled bythe engine inlet air. Alternatively, the transmission system 20 andpropeller 24 may be disposed on the exhaust side of the microturbineengine 12. In this alternate configuration, the transmission system andpropeller must operate in a hot environment, and thus must be designedto endure the increased thermal loading that stems from operation in thehot environment on the exhaust side of the engine 12.

The output shaft 14 of the microturbine 12 operates at very highrotational speeds, typically in the range between 72,000 RPM and 150,000RPM with an output power between 150 HP and 5 HP. For low-speed UAVapplications, the propeller 24 operates at much slower rotationalspeeds, typically on the order of 3700 RPM to 4500 RPM. Theseconstraints result in a required reduction ratio from the microturbineengine RPM to the propeller RPM on the order of 28:1 to 24:1. Thetransmission system 20 provides this required speed reduction over theoutput power range (150 HP to 5 HP) of the microturbine engine.

In the preferred embodiment of the present invention, the transmissionsystem 20 as well as the microturbine engine are of small size and lowweight. Preferably, the maximum diameter of the transmission system isless than 12 inches. Such size and weight constraints are suitable foruse in advanced UAVs. Moreover, the transmission system 20 is preferablyrealized by a two-stage design. There are many different two-stagedesigns that can be used to realize the transmission system 20 asdescribed below with respect to FIGS. 2-8.

FIG. 2 illustrates a first exemplary embodiment of the transmissionsystem 20 of FIG. 1. The two-stage transmission system, which is labeled20′, is an In-line Lay Shaft configuration. The first reduction stage isprovided by a pinion P21 integral to the input shaft 18 and two gears(each labeled G21) that are spaced 180 degrees apart. The radialcenterline joining the pinion P21 and two gears G21 can be in any clockorientation, but is preferably disposed in a horizontal orientation tomake lubrication easier. The second stage includes two pinions P22(which are integral to the two intermediate shafts 51, 53 that includethe two gears G21 of the first stage) and one gear G22 that is affixedto an output shaft. The two pinions P22 of the second stage are spaced180 degrees apart. Since, the gearing forms a “closed-loop”, there mustbe a certain tooth relationship between the gears and pinions of the twostages so that the transmission system can be assembled and rotatewithout interference. The two intermediate shafts 51 and 53, and thespline coupled to the output shaft 22 are supported by bearings asshown.

The reduction ratio of the In-Line Lay Shaft configuration of FIG. 2 isprovided by:${\frac{D_{G\quad 21}}{D_{P\quad 21}}\frac{D_{G\quad 22}}{D_{P\quad 22}}},$where D_(G21) and D_(P21) are the diameters of the pitch circle for theteeth of the gears G21 and pinion P21 of the first stage, and D_(G22)and D_(P22) are the diameters of the pitch circle for the teeth of thegear G22 and the pinions P22 of the second stage.Alternatively, the reduction ratio of the In-Line Lay Shaftconfiguration of FIG. 2 is provided by:${\frac{N_{G\quad 21}}{N_{P\quad 21}}\frac{N_{G\quad 22}}{N_{P\quad 22}}},$where N_(G21), N_(P21), N_(G22), N_(P22) are the number of teeth alongthe pitch circle of the respective pinions and gears.

In the exemplary configuration shown, the diameters D_(G21) and D_(P21)are 3.731 inches and 0.692 inches, respectively, and the teeth countsN_(G21) and N_(P21) are 97 and 18, respectively. These values provide areduction ratio of the first stage on the order of 5.4. Moreover, thediameters D_(G) ₂₂ and D_(P22) are 3.638 inches and 0.785 inches,respectively, and the teeth counts N_(G) ₂₂ and N_(P22) are 88 and 18,respectively. These values provide a reduction ratio of the second stageon the order of 4.6. The reduction ratio of the transmission system 20′is the product of these two reduction values, which is (5.4*4.6):1 andthus on the order of 25:1. With the input shaft 18 rotating at about104,600 RPM with a power on the order of 70 HP, the two intermediateshafts 51,53 are rotating at about 19,410 RPM, and the output shaft 22is rotating at about 4,191 RPM, which is a value suitable for driving apropeller of a small propeller-driven fixed-wing UAV.

Note that the largest diameter of the configuration of FIG. 2 is formedby the first reduction stage, which is dictated by the outside diametersof the two gears G21 and the pinion P21. In the configuration shown,this sum is provided by (2*3.731 inches)+0.692 inches, which is on theorder of 9 inches in diameter. The housing requires an additional inch,thus the largest diameter of the transmission system 20″ is on the orderof 10 inches. The width of the transmission system 20′ is on the orderof 4.6 inches as shown.

FIG. 3 illustrates a second exemplary embodiment of the transmissionsystem 20 of FIG. 1. The two-stage transmission system, which is labeled20″, is an In-line Star-Star configuration. The first stage is providedby a star planetary system having a sun gear S31, a plurality ofplanetary pinions (labeled P31), a fixed carrier C31 operably coupled tothe planetary pinions P31, and an output ring gear R31. Similarly, thesecond reduction stage is provided by a star planetary system having asun gear S32, a plurality of planetary pinions P32, a fixed carrier C32operably coupled to the planetary pinions P32, and an output ring gearR32. The input shaft of the transmission system is affixed to the sungear S31 of the first reduction stage. The output ring gear R31 of thefirst reduction stage is operably coupled to an intermediate shaft 61with the sun gear S32 of the second stage affixed thereto. The outputring gear R32 of the second stage is operably coupled to the outputshaft 22 of the system 20″. The pinions P31 of the first stage, theintermediate shaft 61, the pinions P32 of the second stage, and thespline coupled to the output shaft 22 are supported by bearings asshown.

The reduction ratio of the In-Line Star-Star configuration of FIG. 3 isprovided by:${\frac{N_{R\quad 31}}{N_{S\quad 31}}\frac{N_{R\quad 32}}{N_{S\quad 32}}},$where N_(R31), N_(S31), N_(R32), N_(S32) are the number of teeth alongthe pitch circle of the respective ring gears and sun gears of the twostages.In the exemplary configuration shown, the diameters D_(S31), D_(P31) andD_(R31) of the first stage gears are 0.594 inches, 1,281 inches and3.156 inches, respectively, and the teeth counts N_(S31), N_(P31) andN_(R31) of the first stage gears are 19, 41, and 101, respectively.These values provide a reduction ratio of the first stage on the orderof 5.3. Moreover, the diameters D_(S32), D_(P32) and D_(R32) of thesecond stage gears are 0.769 inches, 1.423 inches and 3.615 inches,respectively, and the teeth counts N_(S32), N_(P32) and N_(R32) of thesecond stage gears are 20, 37, and 94 respectively. These values providea reduction ratio of the second stage on the order of 4.7. The reductionratio of the system 20″ is the product of these two reduction values,which is (5.3*4.7):1 and thus on the order of 25:1. With the input shaft18 rotating at about 104,600 RPM with a power on the order of 70 HP, theintermediate shaft 61 is rotating at about 19,677 RPM, and the outputshaft is rotating at about 4,187 RPM, which is a value suitable fordriving the propeller of a small propeller-driven fixed-wing UAV.

Note that the largest diameter of the configuration of FIG. 3 is formedby the second stage, which is dictated by the outside diameter of thering gear R32 of the second stage. In the configuration shown, thisdimension is on the order of 3.6 inches in diameter. The housingrequires a few additional inches in diameter, thus the largest diameterof the transmission system 20″ is on the order of 6 inches. The width ofthe transmission system 20″ is on the order of 6.4 inches as shown.

Note that it is desirable that the star planetary systems of the firstand second stage satisfy well known “hunting teeth” and “sequencemeshing” constraints. For “hunting teeth”, the ratio (N_(S)/N_(P)) orthe ration (N_(R)/N_(P)) is equal to a whole number plus an irreduciblefraction. For “sequence meshing”, the ratio (N_(R)/# of pinions) or theratio (N_(s)/# of pinions) is equal to a whole number plus anirreducible fraction. Moreover, the number of pinions is selected toavoid interference therebetween by satisfying the following constraint:${\#\quad{of}{\quad\quad}{pinions}} \leq \frac{\pi}{{arc}\quad{\sin\left\lbrack \frac{\left\lbrack {{dp} + {2a}} \right\rbrack}{{ds} + {dp}} \right\rbrack}}$where dp is the pinion pitch diameter, ds is the sun gear pitch diameterand a is the addendum of the pinion.

FIG. 4 illustrates a third exemplary embodiment of the transmissionsystem 20 of FIG. 1. The two-stage transmission system, which is labeled20′″, is an Offset Star-Spur configuration. The first stage is providedby a star planetary system having a sun gear S41, a plurality ofplanetary pinions P41, a fixed carrier C41 operably coupled to theplanetary pinions P41, and an output ring gear R41. The second stage isprovided by a spur pinion P42 and gear G42. The input shaft 18 iscoupled to the sun gear S41 of the first stage. The output ring gear R41of the first stage is integral to an intermediate shift 71 with thepinion P42 of the second reduction stage integral thereto. The gear G42of the second stage is integral to a spline that is operably coupled tothe output shaft 22 of the transmission system 20′″. The pinions P41 ofthe first stage, the intermediate shaft 71, and the spline of the gearG42 of the second stage are supported by bearings as shown.

The reduction ratio of the Offset Star-Spur configuration of FIG. 4 isprovided by:${\frac{N_{R\quad 41}}{N_{S\quad 41}}\frac{N_{G\quad 42}}{N_{P\quad 42}}},$where N_(R41), N_(S41), N_(G42), N_(P42) are the number of teeth alongthe pitch circle of the respective first stage ring gear R41, first sungear S41, second stage pinion P42 and second stage gear G42.In the exemplary configuration shown, the diameters D_(S41), D_(P41) andD_(R41) of the first stage gears are 0.594 inches, 1,281 inches and3.156 inches, respectively, and the teeth counts N_(S41), N_(P41), andN_(R41) of the first stage gears are 19, 41, and 101, respectively.These values provide a reduction ratio of the first stage on the orderof 5.3. Moreover, the diameters D_(P42) and D_(G42) of the second stagegears are 0.950 inches, 4.50 inches, respectively, and the teeth countsN_(P42) and N_(G42) of the second stage gears are 19 and 92,respectively. These values provide a reduction ratio of the second stageon the order of 4.8. The reduction ratio of the transmission system 20′″is the product of these two reduction values, which is (5.3*4.8):1 andthus on the order of 25:1. With the input shaft rotating at about104,600 RPM with a power on the order of 70 HP, the intermediate shaft71 is rotating at about 19,677 RPM, and the output shaft 22 is rotatingat about 4,154 RPM, which is a value suitable for driving the propellerof a small propeller-driven fixed-wing UAV.

Note that the largest diameter of the configuration of FIG. 4 is formedby the second stage, which is dictated by the diameter of the secondstage gear G42. In the configuration shown, this dimension is on theorder of 6.5 inches in diameter. The housing requires a few additionalinches in diameter, thus the largest diameter of the transmission system20 ′″ is on the order of 8.8 inches. The width of the transmissionsystem 20′″ is on the order of 5.3 inches as shown.

Note that it is desirable that the star planetary system of the firststage satisfy well known “hunting teeth” and “sequence meshing”constraints as described above. Moreover, the number of pinions isselected to avoid interference therebetween as described above.

FIGS. 5A and 5B illustrate a fourth exemplary embodiment of thetransmission system 20 of FIG. 1. The two-stage transmission system,which is labeled 20″″, is an Offset Compound Idler configuration. Thefirst stage includes a floating input pinion P51 driving twodiametrically opposed gears G51. Each gear G51 is connected to acorresponding intermediate shaft 81 with a second stage pinion P52 thatcombines to drive a single output gear G52. This arrangement is verysimilar to the lay shaft arrangement described above with respect toFIG. 2, except that the output gear G52 of the second stage is offsetfrom the input. This offset is possible geometrically because the centerdistance of the second stage is larger that the center distance of thefirst stage. The intermediate shafts for the two gears G51 and twopinions P52 (one shown as 81 in the cross section of FIG. 5B), and thespline coupled to the output shaft 22 are supported by bearings asshown.

The reduction ratio of the Offset Compound Idler configuration of FIG. 5is provided by the same formulas as the Inline Lay Shaft configurationdescribed above with respect to FIG. 2.

In the exemplary configuration shown, the diameters D_(G51) and D_(P51)are 2.767 inches and 0.60 inches, respectively, and the teeth countsN_(G51) and N_(P51) are 83 and 18, respectively. These values provide areduction ratio of the first stage on the order of 4.6. Moreover, thediameters D_(G52) and D_(P52) are 4.020 inches and 0.741 inches,respectively, and the teeth counts N_(G52) and N_(P52) are 103 and 19,respectively. These values provide a reduction ratio of the second stageon the order of 5.4. The reduction ratio of the system 20″″ is theproduct of these two reduction values, which is (4.6*5.4):1 and thus onthe order of 25:1. With the input shaft 18 rotating at about 104,600 RPMwith a power on the order of 70 HP, the two intermediate shafts 81 arerotating at about 22,684 RPM, and the output shaft 22 is rotating atabout 4,184 RPM, which is a value suitable for driving the propeller ofa small propeller-driven fixed-wing UAV.

Note that the cross section of FIG. 5B is taken through the mesh path asnoted in FIG. 5A This gives the appearance of a large assembly. But, infact, the intermediate shafts are on the same centerline as the inputpinion so that the overall height of the transmission system is on theorder of 7.5 inches. The width of the transmission system is on theorder of 4.6 inches as shown. It is expected that weight and costs ofcompound idler configuration of FIGS. 5A and 5B will be lower relativeto the other configurations discussed herein. Thus, it is expected thatthe compound idler configuration will be advantageous for use inlightweight applications, such as in small fixed-wing propeller-drivenUAVs.

FIGS. 6A and 6B illustrate a fifth exemplary embodiment of thetransmission system 20 of FIG. 1. The two-stage transmission system,which is labeled 20′″″, is an Inline Traction-Internal Gearconfiguration. The first stage is realized by a traction drive. Atraction drive utilizes rollers (not a teeth mesh) to transfer energy.It works on the principle of creating a normal force between two rollersthat can support a tangential load equal to the normal force times thetraction coefficient. The traction coefficient is similar to thecoefficient of friction. The traction drive is cooled and lubricated byspecially developed traction fluids. The traction fluid, in combinationwith the rolling elements, acts like a spur gear, which has the benefitof a damping effect on the transmission. The shearing force of anelasto-hydrodynamic lubrication oil film between the two rotatingsurfaces achieves traction drive. An automatic speed/load adjustmentdevice obtains high efficiency by providing the correct amount of radialforce to permit drive. The radial force automatically adjusts the torquerequired. The traction drive is typically lower in cost than a gearedreduction stage because it avoids the costs of a teeth mesh.

As shown in FIG. 6A, two of the three pinions (labeled P61 _(A), P61_(B)) of the traction stage are fixed in position and one of the threepinions (labeled P61 _(C)) is loose by a small amount of space (forexample, 0.0031 inches). The center of the posit ion of the loose pinionP61 _(C) on the carrier is offset in a critical direction such that whentorque is applied to the traction sun S61, the loose pinion P61 _(C)attempts to move away from its position at rest and the contact linebetween the inside roller and the post will move away from thecenterline between the traction sun S61 and the loose post member. Thiscreates a larger normal force and preloads all three pinion rollers P61_(A), P61 _(B), P61 _(C). The adjustment varies with torque such thatthe correct amount of normal force is provided to transmit the appliedtorque. As torque increases, the loose roller P61 _(C) automaticallyadjusts to a new position and provides the correct new normal forcerequired.

In the configuration shown, there is a traction ring R61. But, its onlypurpose is to provide a radial load reaction member and thus it merelyspins along without torque. The three traction pinions P61 _(A), P61_(B), P61 _(C) are attached to a carrier C61 through bearings such thatthey can rotate about there own centers, and the carrier C61 is fixed inposition. Integral to each traction pinion is an external spur pinionP62 (one shown in the cross-section of FIG. 6B). The three externalpinions P62 drive an output internal ring gear R62 that drives theoutput shaft 22. The three external pinions P62 and the output internalring gear R62 provide the second stage of the transmission system 20′″″.

The reduction ratio of the Inline Traction-Internal Gear configurationof FIGS. 6A and 6B is provided by:${\frac{D_{P\quad 61}}{D_{S\quad 61}}\frac{D_{R\quad 62}}{D_{P\quad 62}}},$where D_(P61) and D_(S61), are the diameters of the roller for the teethof the respective pinions P61 and sun D61 of the first stage tractiondrive, and D_(R62) and D_(P62) are the diameters of the pitch circle forthe teeth of the respective ring gear R62 and pinions P62 for the secondstage.Alternatively, the reduction ratio of the Inline Traction-Internal Gearconfiguration of FIGS. 6A and 6B is provided by:${\frac{D_{P\quad 61}}{D_{S\quad 61}}\frac{N_{R\quad 62}}{N_{P\quad 62}}},$where D_(P61) and D_(S61), are the diameters of the roller for the teethof the respective pinions P61 and sun S61 of the first stage tractiondrive, and N_(R62) and N_(P62) are the number of teeth of the pitchcircle for the respective ring gear R62 and pinions P62 for the secondstage.In the exemplary configuration shown, the diameters D_(P61) and D_(S61)are 1.680 inches and 0.40 inches, respectively. These values provide areduction ratio of the first stage traction unit on the order of 4.Moreover, the diameters D_(R62) and D_(P62) are 2.50 inches and 0.421inches, respectively, and the teeth counts N_(R62) and N_(P62) are 95and 16, respectively. These values provide a reduction ratio of thesecond stage on the order of 5.9. The reduction ratio of thetransmission system is the product of these two reduction values, whichis (4*5.9):1 and thus on the order of 24:1. With the input shaft 18rotating at about 104,600 RPM with a power on the order of 70 HP, thepinions P61 are rotating at about 24,905 RPM, and the output shaft 22 isrotating at about 4,195 RPM, which is a value suitable for driving thepropeller of a small propeller-driven fixed-wing UAV.

Note that the overall height of the transmission system is on the orderof 4.4 inches. The width of the transmission system is on the order of3.3 inches as shown.

FIG. 7 illustrates a sixth exemplary embodiment of the transmissionsystem 20 of FIG. 1. The two-stage transmission system, which is labeled20″″″, is an Inline Traction-Planetary Gear configuration. The firststage is realized by a traction drive stage as described above withrespect to FIGS. 6A and 6B, and the second stage is provided by a starplanetary system having a sun gear S72, a fixed ring gear R72, and aplurality of planetary pinions P72 operably coupled to a rotatingcarrier C72 that drives the output shaft 22 of the transmission system.In this configuration, the traction ring R71 of the first stage is usedto drive the sun gear S72 of the second stage.

The reduction ratio of the Inline Traction-Planetary Gear configurationof FIG. 7 is provided by:${\frac{D_{R\quad 71}}{D_{S\quad 71}}\left( {1 + \frac{D_{R\quad 72}}{D_{S\quad 72}}} \right)},$where D_(R71) and D_(S71), are the diameters of the roller for the teethof the respective ring R71 and sun S71 of the first stage tractiondrive, and D_(R72) and D_(S72) are the diameters of the pitch circle forthe teeth of the respective ring gear R72 and sun gear S72 for thesecond stage.Alternatively, the reduction ratio of the Inline Traction-Internal Gearconfiguration of FIGS. 6A and 6B is provided by:$\frac{D_{R\quad 71}}{D_{S\quad 71}}\left( {1 + \frac{N_{R\quad 72}}{N_{S\quad 72}}} \right)$where D_(R71) and D_(S71), are the diameters of the roller for therespective ring R71 and sun S71 of the first stage traction drive, andN_(P72) and N_(S72) are the number of teeth of the pitch circle for therespective ring gear R72 and sun gear S72 for the second stage.

In the exemplary configuration shown, the diameters D_(R71) and D_(S71)are 3.775 inches and 0.675 inches, respectively. These values provide areduction ratio of the first stage traction unit on the order of 5.6.Moreover, the diameters D_(R72) and D_(S72) are 3.64 inches and 1.16inches, respectively, and the teeth counts N_(R72) and N_(S72) are 91and 29, respectively. These values provide a reduction ratio of thesecond stage on the order of 4.1 The reduction ratio of the transmissionsystem is the product of these two reduction values, which is(5.6*4.1):1 and thus on the order of 23:1. With the input shaft 18rotating at about 104,600 RPM, the first stage ring R1 is rotating atabout 17,318 RPM, and the output shaft 22 is rotating at about 4,185RPM, which is a value suitable for driving a propeller of a smallpropeller-driven fixed-wing UAV.

Note that the overall height of the transmission system 20″″″ is on theorder of 4.3 inches, and the width of the transmission system is on theorder of 3.5 inches as shown.

Another consideration for the design of the transmission system of thepresent invention is the high speed operation of the input shaft 18.Designing bearings to operate at such high speeds in challenging.However, it is very difficult to designs bearings suitable for use withradial and/or thrust loads at such high speeds. Thus, it is an objectiveto have the input shaft of the transmission carry only torque and haw noradial or thrust loads (and if possible have no bearings). In order toreduce the radial and thrust loads on the input shaft of thetransmission, it is preferable that the first reduction stage of thetransmission system provide for self-equilibration. This condition isprovided by equally spacing apart the gear or roller outputs from agiven pinion such that the resultant load on the pinion is cancelledout. FIGS. 8A and 8B illustrates the physics provided by aself-equilibrating configuration. For example, consider the two gearsystem of FIG. 8A. Suppose the tangential tooth load on the upper gearis higher than on the lower gear. The radial tooth loads are theresultant of the tangential tooth loads; therefore, the radial toothload on the upper gear will also be higher than on the lower. Thisdifference in load will force the pinion into mesh on the lighter loadside (e.g., lower gear side) until the loads are exactly balanced.Hence, if the pinion is permitted to “float” such that is trapped byequally spaced surrounding gears or rollers, the loads are equalizedresulting in the desired cancellation. In this configuration, the inputdrive shaft 18 of the transmission system need not be supported by highbearings, thereby eliminating the requirement of loaded bearings at highspeeds.

Also note that when a planetary system is used in the first stage of thetransmission system 20, it should be realized with a fixed carrier. Thisis necessary to avoid centrifugal forces acting on the pinions of arotating carrier, which can substantially reduce bearing life.

Preferably, the input shaft 18 of the transmission system 20 is coupledto the output shaft 14 of the microturbine 12 via an outside diameterpiloted spline coupling mechanism (sometimes referred to as a “flat rootmajor diameter fit spline coupler”) as shown in FIG. 9. In thisconfiguration, the input shaft 16 of the transmission system includes anoutside diameter piloted spline section 91. The output shaft 14 of themicroturbine 12 includes projections 93 that project radially inwardfrom the inner diameter surface to engage the piloted section 91. A snapring 95 or other suitable retention mechanism is used to retain theinput shaft 95 in the horizontal direction as shown. Alternatively, theexternal spline surface can be provided on the output shaft 14 of themicroturbine engine 12 with an internal spline surface provided on theinput shaft 16 of the transmission system.

An accessory unit may be operably coupled to the drive train of thetransmission system. For example, the accessory unit may be astarter/generator having a brushless 4-pole permanent magnet AC typearchitecture with a plurality (e.g., 4) magnets mounted around a rotorperimeter. A power control unit converts the alternating current outputto direct current output in generating mode, and converts the directcurrent input to alternating current input in starting mode. Theaccessory unit can be coupled to the drive train of the transmissionsystem in many ways.

For example, the accessory can be directly mounted on the input shaft ofthe transmission system (or the output shaft of the microturbineengine). This configuration may be problematic in designs that rely on afloating input shaft for the purposes of self-equilibrating load sharingand radial load cancellation as described above due to the weight andany imbalance of the accessory on the input shaft of the transmissionsystem. On the other hand, for a design where a traction-type drive isused in the first stage of the transmission systems the accessorymounted onto transmission input shaft will have very little influence onload sharing due to the fact that the radial loads of the traction driveare greater than ten times the tangential loads, and the three equallyspaced traction pinions rigidly hold the traction sun in place. Theradial rigidity of the traction sun can easily handle any influence ofthe accessory mounted on the input shaft.

Alternatively, the accessory unit may be mounted on a separate mountingpad and driven by the drive train of the transmission system. Forexample, consider the Offset Compound Idler configuration of FIGS. 5Aand 5B. In this configuration, a pinion may be integral to the rotatingshaft of the accessory unit and driven by an idler gear meshed to one ofthe gears G1 of the first stage. Preferably, the number of teeth of thepinion that is integral to the rotating shaft of the accessory unit isequal to the number of teeth of the pinion P1 of the first stage toenable the rotating shaft of the accessory unit to be driven at the samerotational speed as the input shaft 18 of the transmission system 20.Moreover, the idler gear provides clearance between the accessory mountand the stage(s) of the transmission system.

The transmission system also requires a lubrication system. Preferably,the lubrication system includes a conventional oil filter and pumpsystem. The oil pump may be a vane-type pump, gear pump, or a Gerotorpump, which are all well known. In addition, the lubrication systempreferably includes an oil cooler device as is well known. In the eventthat the oil type, temperature and pressure requirements of the engineand transmission system are similar, the lubrication system of thetransmission system may be combined with the lubrication system of theengine as is well known.

For fixed-wing applications, the output shaft of the transmission systemis coupled to a propeller. The gyroscopic moment induced by the expectedpitch rate and yaw rate and mass moment of inertia of the propellerdictates the size of the output shaft of the transmission system. Anexemplary mechanism for coupling the output shaft of the transmissionsystem to a propeller is shown in FIG. 10. Note that the couplingmechanism may be integrated into the housing of the transmission systemfor a compact design.

The exemplary embodiments of the transmission system (andmicroturbine-based propulsion system employing such transmissionsystems) described above are suitable for use in small fixed-wingaircraft applications such as small UAVs. The transmission systems (andmicroturbine-based propulsion system employing such transmissionsystems) can also be readily adapted for use in other aircraftapplications, such as in small vertical lift aircraft applications asshown in FIG. 11A. In such applications, a power plant 110 suitable foruse in propelling a vertical lift aircraft includes a microturbineengine 112 with an output shaft 114. A coupling mechanism 116 couplesthe output shaft 114 to the input shaft 118 of a transmission system120. The transmission system 120 operates to reduce the speed of theoutput shaft 114 of the microturbine engine 112 at its own output shaft122. The output shaft 122 of the transmission system 120 is coupled to abevel gear assembly 124 or other suitable drive mechanism that transmitsthe power of the rotating shaft 122 to a rotating vertical shaft 126. Arotor 128 is coupled to the rotating vertical shaft 126. The rotor 128,when driven by the microturbine engine 112 and transmission system 120,provides thrust that propels an aircraft body, such as the body of thevertical lift aircraft 130 of FIG. 11B. Note that in the configurationshown, the transmission system 120 and bevel gear assembly 124 aredisposed on the intake side of the microturbine engine 112. Thisconfiguration allows the transmission system 120 and bevel gear assembly124 to be cooled by the engine inlet air. Alternatively, thetransmission system 120 and bevel gear assembly 124 may be disposed onthe exhaust side of the microturbine engine 112. In this alternateconfiguration, the transmission system 120 and bevel gear assembly 124must operate in a hot environment, and thus must be designed to endurethe increased thermal loading that stems from operation in the hotenvironment on the exhaust side of the engine 112.

The output shaft 114 of the microturbine 112 operates at very highrotational speeds, typically in the range between 72,000 RPM and 150,000RPM with an output power between 150 HP and 5 HP. For UAV applications,the rotor 128 operates at much slower rotational speeds, typically onthe order of 3700 RPM to 4500 RPM. These constraints result in arequired reduction ratio from the microturbine engine RPM to the rotorRPM on the order of 28:1 to 24:1. The transmission system 120 and thebevel gear assembly 124 provide this required speed reduction over theoutput power range (150 HP to 5 HP) of the microturbine engine.

In the preferred embodiment of the present invention, the transmissionsystem 120 as well as the microturbine engine 112 is of small size andlow weight. Preferably, the maximum diameter of the transmission system120 is less than 12 inches. Such size and weight constraints aresuitable for use in advanced UAVs.

Moreover, the transmission system 120 may be realized by a two-stagedesign. There are many different two-stage designs that can be used torealize the transmission system 120 as described above with respect toFIGS. 2-8. Note that the bevel gear assembly 124 will typically providea reduction ratio on the order of 2:1 to 3:1. Therefore, the reductionratio of the two stage transmission designs of FIGS. 2-8 as describedabove are readily adapted to provide for a lower reduction ratio (e.g.,on the order of 9:1) to provide a total reduction ratio on the order of28:1 to 24:1. Preferably, the reduction ratio of each stage of thetwo-stage transmission designs of FIGS. 2-8 are on the order of 3:1.

Note that for the traction drive designs of FIGS. 6A-6B and 7, the firststage traction planetary can be readily adapted to provide any reductionratio up to 13:1 in the single traction stage. Thus, for vertical liftapplications, the second geared planetary stage may be completelyeliminated, thereby providing for significant cost advantages.

There have been described and illustrated herein several embodiments ofa small-size high-speed transmission system and microturbine-basedpropulsion systems utilizing the improved transmission system. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. Thus, while particular shaft speeds,horsepowers and reduction ratios have been disclosed, it will beappreciated that the transmission systems described herein can bereadily adapted for use in a broad range of shaft speeds, horsepowersand reduction ratios. For example, the transmission systems describedherein may be readily adapted for use with microturbines that operate inan extended range between 50,000 RPM and 200,000 RPM with an outputpower between 200 HP and 5 HP. In these systems, the reduction ratioprovided by the transmission system will likely be increased for certainapplications, such as the UAV applications described herein. Inaddition, while particular types of transmission stages have beendisclosed, it will be understood that other well know transmission stagedesigns can be used. For example, and not by way of limitation, thetransmission system can be realized by a harmonic drive stage. Theharmonic drive includes three basis elements (a circular spline, aflexspline, and a wave generator) that utilize non-circular rotation ofthe flexspline to drive the circular spline. Also, while thetransmission system and microturbine-based propulsion system of thepresent invention are preferably used in conjunction with a propeller topropel a small-size fixed-wing aircraft, it will be appreciated that itwill be readily adapted for other small-size aircraft applications, suchas vertical lift aircraft or hybrid tilt-rotor aircraft. Moreover, thetransmission system and microturbine-based propulsion system of thepresent invention may be readily adapted for use in other applications,such as marine propulsion systems, automotive applications, electricalpower generations applications, micro-turbine based HVAC applicationsand hydraulic applications. Finally, while microturbine-based propulsionsystems of the present invention may consume a wide variety of fuels,including liquid fuels (such as liquefied natural gas) or gaseous fuels(such as natural gas or propane). It will therefore be appreciated bythose skilled in the art that yet other modifications could be made tothe provided invention without deviating from its spirit and scope asclaimed.

1. A transmission system for use with a microturbine engine having anoutput shaft, the transmission system comprising: a first shaft operablycoupled to the output shaft of the microturbine engine; and tworeduction stages that drive a second shaft at a reduced rotation speedwith respect to said first shaft.
 2. A transmission system according toclaim 1, wherein: the output shaft of the microturbine engine operatesat a rotational speed in a range between 72,000 RPM and 150,000 RPM withan output power between 150 HP and 5 HP.
 3. A transmission systemaccording to claim 2, wherein: said two reduction stages provide areduction ratio having a value of at least
 19. 4. A transmission systemaccording to claim 3, wherein: said reduction ratio has a value greaterthan
 24. 5. A transmission system according to claim 1, wherein: largestdiameter of said transmission system is less than twelve inches.
 6. Atransmission system according to claim 1, wherein: said second shaft ofsaid transmission system is operably coupled to a rotating element forpropelling an aircraft body.
 7. A transmission system according to claim6, wherein: said aircraft body comprises a fixed-wing aircraft body, andsaid rotating element comprises a propeller.
 8. A transmission systemaccording to claim 6, wherein: said aircraft body comprises a verticallift aircraft body, and said rotating element comprises a rotor.
 9. Atransmission system according to claim 1, wherein: the output shaft ofsaid microturbine engine is coupled to said first shaft by an outsidediameter piloted spline coupling mechanism.
 10. A transmission systemaccording to claim 9, wherein: an outside diameter section of said firstshaft includes a plurality of pilot grooves that engage projections thatextend radially downward from a corresponding inside diameter section ofthe output shaft of the microturbine engine.
 11. A transmission systemaccording to claim 1, wherein: at least one said two stages comprises ageared planetary system.
 12. A transmission system according to claim11, wherein: each of said two stages comprises a geared planetarysystem.
 13. A transmission system according to claim 1, wherein: one ofsaid two stages comprises a traction drive system including a planetarysystem of rollers.
 14. A transmission system according to claim 13,wherein: the other of said two stages comprises a geared drive systemincluding a planetary system of gears.
 15. A transmission systemaccording to claim 1, wherein: said two stages comprise one of: i) anin-line lay shaft configuration; ii) an in-line star-star configuration;iii) an offset star-spur configuration; iv) an offset compound idlerconfiguration; v) an inline traction-internal gear configuration; andvi) an inline traction-planetary gear configuration.
 16. A transmissionsystem according to claim 1, wherein: said two stages comprise aself-equilibrating input stage.
 17. A transmission system according toclaim 16, wherein: said self-equilibrating input stage includes afloating pinion integral to said first shaft.
 18. A transmission systemaccording to claim 17, wherein: said first shaft is bearingless.
 19. Atransmission system according to claim 1, wherein: said two stagescomprises a first reduction stage operably coupled to the first shaftthat is followed by a second reduction stage operably coupled to thesecond shaft, wherein said second reduction stage provides a reductionratio greater than said first reduction ratio.
 20. A propulsion systemfor an aircraft comprising: a microturbine engine having an outputshaft; and a transmission system according to claim
 1. 21. A propulsionsystem according to claim 20, wherein: said second shaft of saidtransmission system is operably coupled to a rotating element forpropelling an aircraft body.
 22. A propulsion system according to claim21, wherein: said aircraft body comprises a fixed-wing aircraft body,and said rotating element comprises a propeller.
 23. A propulsion systemaccording to claim 21, wherein: said aircraft body comprises a verticallift aircraft body, and said rotating element comprises a rotor.
 24. Anaircraft comprising: an aircraft body; the propulsion system of claim20; and a rotating element, operably coupled to said propulsion systemfor propelling said aircraft body.
 25. An aircraft according to claim24, wherein: said aircraft body comprises a vertical lift aircraft body,and said rotating element comprises a rotor.
 26. An aircraft accordingto claim 24, wherein: said aircraft body comprises a fixed-wing aircraftbody, and said rotating element comprises a propeller. 27-47. (canceled)